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Use an image at least 1200 × 630 pixels for a Substack post thumbnail (the control Substack calls the social preview). Substack’s current dimensions guidance also lists a 14:10 aspect ratio for preview images. Those specifications conflict: 1200 × 630 is about 1.90:1, while 14:10 is 1.40:1. Because Substack does not explain which context accounts for the difference, prepare the 1200 × 630 minimum, keep important content away from the edges, and inspect the preview before publishing.
The correct Substack thumbnail size in 2026
For a post’s social preview, create a file that is at least 1200 × 630 pixels. A larger image with the same general landscape shape gives you room to scale down without making text or faces soft.
Substack’s image-dimensions guidance also states 14:10 as the preview-image aspect ratio. These figures are not equivalent: 1200 × 630 simplifies to approximately 1.90:1, whereas 14:10 is 1.40:1. The published guidance does not say whether the ratio applies to a different display context, nor does it identify which value should override the other.
The practical approach is to treat 1200 × 630 as the minimum pixel requirement, design with a safe central area, and use Substack’s preview to verify the result. Do not stretch a 14:10 source to 1200 × 630; that changes the composition. Crop or reframe instead.
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What “thumbnail” means on Substack
Substack labels the image used when a post is represented outside the article as its social preview. It is separate from the image elements used elsewhere in a publication:
| Image use | Current Substack guidance | How to apply it |
|---|---|---|
| Post thumbnail / social preview | At least 1200 × 630 px; the same dimensions guidance lists 14:10 | Use 1200 × 630 px or larger, then check the crop |
| Publication cover | At least 600 × 600 px | Use this only for the publication’s cover artwork |
| Email banner | Recommended 1100 × 220 px; transparent background specified | Do not substitute this for a post thumbnail |
| Full-width image inside a post | Images taller than a 1:1 height ratio are automatically cropped to 1:1 when set full width | This editor behavior is unrelated to the social-preview minimum |
Substack supports AVIF, GIF, JPG/JPEG, PNG and WebP for post images. The guidance reviewed does not establish a maximum upload file size, so choose a sensible export size and confirm that the editor accepts your file.
How to set a Substack post thumbnail
- Open the draft in Substack’s web editor. The relevant controls are in the post editor rather than in a general image-editing menu.
- Select Settings at the bottom of the draft.
- Find the social preview section. Choose an image already inserted in the post, or select Upload New to add a separate thumbnail.
- Review the preview on the Publish page. Substack saves changes made there automatically; a separate Save button does not appear.
- Publish only after checking the crop. Look at faces, logos, headline text and other details near the edges. If any of them are cut off, return to the source image and reframe it.
The social-preview setting is independent of the images readers see inside the article. You can use one image in the body and a different, more legible composition for the thumbnail.
Designing a 1200 × 630 thumbnail that survives cropping
Keep the focal point central
Center the subject, face or product rather than placing it against the extreme left or right edge. Unknown social layouts can trim sides, and a centered composition gives you the most tolerance.
Use short, high-contrast text
Thumbnail text should remain readable at a small display size. Use a strong contrast between type and background, avoid thin strokes, and leave generous padding. Do not put a URL, byline or essential qualifier on the outermost edges.
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Build a safe area
As a working design rule, reserve an inner margin around the perimeter for critical content. This is not a published Substack pixel specification; it is a way to make the image resilient when a preview is displayed at a different crop.
Check faces and logos twice
A crop that removes a forehead, eyes, logo mark or product label can change the meaning of the image. Inspect the actual preview rather than relying only on the canvas in your graphics application.
Changing the crop in the Website editor
Substack provides crop controls for publication surfaces. From the publication Dashboard, open Website editor, choose Home page, and locate Cropping. You can choose:
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- Smart: focuses on what Substack identifies as the most interesting part.
Use center when your composition is deliberately balanced. Try smart when the important subject is offset, then verify that it has not removed text or a secondary subject. These controls affect the way the image is cropped in the website editor; they do not change the underlying 1200 × 630 file.
Thumbnail versus images inside the post
Do not apply social-preview dimensions to every image in an article. In the web post editor, body images can be resized with the black side bars or changed to Set full width from the image menu. Substack says a full-width image taller than a 1:1 height ratio is automatically cropped to 1:1. That rule concerns the in-post display, not the social preview.
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Many body-image editing features are available only in Substack’s web post editor, not the mobile app editor. If you need resizing or full-width controls, use the web editor.
A practical export checklist
- Canvas is at least 1200 × 630 px.
- The file uses a supported format: AVIF, GIF, JPG/JPEG, PNG or WebP.
- Important faces, logos and words sit inside a generous central safe area.
- Text remains legible when the image is reduced to a small card.
- You have not confused a 600 × 600 publication cover or 1100 × 220 email banner with the post thumbnail.
- You inspected the social preview after uploading it.
- You checked the published page, not just the draft canvas, if the crop is especially important.
Common problems and fixes
The image looks stretched
Cause: a 14:10 composition was forced into 1200 × 630 (or the reverse) without cropping. Fix: return to the original, crop deliberately around the subject, and export a new image rather than distorting it.
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Cause: the focal content was too close to an edge, or the selected crop favors another part of the image. Fix: move the content inward, upload the revised file, and test center versus smart cropping in the Website editor.
The preview still shows the old image
Cause: the new file was selected in the body but not in the social-preview setting. Fix: open Settings at the bottom of the draft, choose the social preview image explicitly, and verify the Publish page preview. Changes there save automatically.
The body image is unexpectedly square
Cause: it was set to full width and is taller than Substack’s 1:1 height limit. Fix: use the image menu to return to a regular width, or prepare a shorter crop for the full-width presentation.
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The mobile app does not show the editing control
Cause: some image-editing features are web-editor only. Fix: open the draft in a desktop browser and make the change there.
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For a one-off post, Substack’s own preview is usually enough. For a publishing workflow, you can capture the public post URL after publishing and compare the rendered result with your source artwork. A screenshot records what a browser actually displays, including the crop and any responsive layout changes.
Or skip the browser setup
ScreenshotNeo can capture a public post or preview URL with one request. Before the capture it accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups and chat widgets; each cleanup step can be disabled. Bot checks, CAPTCHAs, blank pages, timeouts, failed loads and cache hits are not billed, and the response identifies the result with X-Page-Verdict and X-Billed headers. Its MCP server provides take_screenshot, get_page_info and capture_pdf tools for Claude, Cursor and other MCP clients.
See the complete parameter reference in the ScreenshotNeo documentation. This cURL request captures a published URL as a WebP file:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://substack.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://substack.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
const q = new URLSearchParams({ access_key: 'YOUR_API_KEY', url: 'https://substack.com' });
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);Replace the example URL with your published Substack post URL. ScreenshotNeo supports full-page capture, device presets and custom viewports, so you can inspect the preview at the size your readers are likely to see. One thousand screenshots per month are free with no card; paid plans start at $5 for 3,000. Create a free ScreenshotNeo account.
Cost and workflow considerations
Substack’s documented dimensions are platform guidance, not a guarantee that every external service will display an identical crop. Keep the original high-resolution artwork so you can re-export when the composition changes. For teams, store the source file, the exported 1200 × 630 version and the final preview screenshot together; that makes it easier to diagnose whether a problem came from the artwork or the rendered page.
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If you automate captures, wait for the page to finish loading before judging the image. A consent dialog, bot check or timeout can produce a screenshot that is not the article you intended to review. ScreenshotNeo exposes the page verdict and billing status in response headers, which lets a script distinguish a clean capture from a failed load.
Frequently Asked Questions
Does Substack require exactly 1200 × 630 pixels?
No. The current guidance says at least 1200 × 630 pixels, so larger files are allowed. Keep the composition compatible with that landscape minimum and verify the preview.
Should I use 14:10 or 1200 × 630?
Substack lists both, but they are different proportions and the guidance does not reconcile them. Use 1200 × 630 as the actionable minimum, avoid edge placement, and inspect the crop.
Can I use the publication cover as the post thumbnail?
You can select an image from the post or upload a new one in the social-preview setting. A publication cover has separate guidance of at least 600 × 600 pixels, so it may need a different crop for a post.
Is a screenshot of the preview the same as the original thumbnail file?
No. The original is the uploaded artwork; a screenshot records how the published page renders it. Use the screenshot for visual QA, not as a replacement master file.
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